Display devices which employ properties such as optical anisotropy and dielectric anisotropy of liquid crystal materials are widely used for watches, desktop calculators, and others. As liquid crystal phase, nematic liquid crystal phase, smectic liquid crystal phase, and cholesteric liquid crystal phase are known, and nematic phase is most general for practical purposes. As the display mode in this case, there exist TN (twisted nematic) mode, DS (dynamic scattering) mode, guest host mode, and DAP (deformation of aligned phases) mode. Whereas many compounds used for these types have heretofore been developed, there is no case in which a single compound is filled as it is in a display device and actually used. As liquid crystal materials used for display devices, it is necessary that the materials exhibit a liquid crystal phase at a wide temperature range with room temperature being its center, are stable against moisture, light, heat, air, electric field, and electromagnetic radiation under the atmosphere in which the materials are used, and have physical properties sufficient to drive display devices. However, these requirements can not be satisfied with a single compound. Then, a method has been adopted in which several kind of liquid crystalline compounds and furthermore non liquid-crystalline compounds are mixed to prepare a composition which meets the requirements, and then supplied for actual uses. Physical properties such as optical anisotropy, dielectric anisotropy, and electrical conductivity required of liquid crystal compositions depends on display mode and shape of devices. In STN mode which is used particularly in recent years for displays of liquid crystal display devices of a high quality, the ratio of elastic constants (k.sub.33 /k.sub.11) of liquid crystal compositions must be enhanced to obtain a good display having a high steepness. Then, compounds which have a high elastic constant, wide range of liquid crystal temperature, and high miscibility with other liquid crystals, are stable, and have a low viscosity not to lose a high response speed have become an important question.
As examples of compounds having a high ratio of elastic constants, alkenyl compounds (Laid-open Japanese Patent Publication No. Sho 59-176,221) expressed by formula (1) are widely known. However, since the temperature range of nematic liquid crystal of the compounds is narrow, compounds having a high clearing point must be further added so that the narrow temperature range of nematic liquid crystal can be compensated when the compounds of formula (1) are mixed with a composition. However, since the compounds having a high clearing point generally exhibit a high viscosity, addition of the compounds increases viscosity of the whole of composition as the result. Derivatives of cynnamonitrile (Laid-open Japanese Patent Publication No. Sho 55-9,012) expressed by formula (2) are also known as liquid crystalline compound. However, since the compounds are chemically unstable, they can hardly be said to be ones of practical use. Whereas cyanocyclohexane derivatives in which cyano group substituted directly in cyclohexane ring (Journal of Molecular Crystal Liquid Crystal, No. 111, page 329, 1984) and which are expressed by formula (3) are also known, the compounds have a high viscosity and a narrow temperature range of liquid crystal. Further, new compounds such as difluorovinyl derivatives (Laid-open Japanese Patent Publication No. Hei 1-175,947) expressed by formula (4) and halogenized vinyl derivatives (WO 93/07,234) expressed by formula (5) have been developed. Whereas the former have a low viscosity, they are poor in reliability and have only a narrow liquid crystal range. The latter have a medium extent of the range of liquid crystal, but they are also poor in reliability and thus they have no practical uses. Then, discovery and development of new materials exhibiting a large elastic constants, and having a high clearing point, low viscosity, and high reliability have been expected. ##STR3##